1. Field of Invention
This invention relates to marking devices and, in particular, to a media transport assembly and method in which a media sheet is pushed and/or pulled by step advancing drive assemblies in a highly repeatable manner against the action of a vacuum force applied to hold the media sheet flat for accurate scanned marking.
2. Description of Related Art
Consumers continue to demand marking devices with increased capabilities. For example, consumers now desire a low cost alternative to conventional laser color printers. Marking devices using other technologies (e.g., ink jet printing) potentially provide such a low-cost alternative, but must be configured to operate at speeds comparable to the competing laser printers.
In marking devices with one or more moving print elements that reciprocate across the width of a media sheet (i.e., from left to right or from right to left) in passes, the media sheet is marked in swaths equal to the length of the print element(s). The operating speed of such a marking device can be increased by increasing the size to the swath marked in each pass. With a larger swath size, fewer passes are required to mark each media sheet.
As the print swath area increases, the area of the marking zone must be increased. The marking zone is defined as the area of the media sheet available for marking in the current swath. The marking zone extends between the entrance drive assembly immediately upstream of the print element(s) to the exit drive assembly immediately downstream of the print element(s). With an increased marking zone area, maintaining the flatness of the media sheet and providing a highly accurate incremental media advance without increasing lead and/or trail edge borders to ensure accurate marking, becomes more difficult. Because the gap between the print element(s) and the media sheet is small (on the order of 1.1 mm), it is not possible to secure the media sheet from the reverse side. In typical marking devices with moving print elements, sufficient flatness and advance accuracy of the media sheet is a concern in leading edge or trailing edge marking situations.
In leading edge or trailing edge marking situations, only one edge of the media sheet is secured and driven by the entrance drive assembly or the exit drive assembly. In contrast, when middle portions of the media sheet are being marked, the leading portion is secured by the exit drive assembly and the trailing portion is secured by the entrance drive assembly. Thus, the flatness of the media sheet and the advance accuracy must be ensured by appropriate tensioning between the entrance drive assembly and the exit drive assembly.
Accordingly, it would be desirable to provide a media transport system capable of advancing at a sufficient speed and accuracy, while ensuring that the media sheet is sufficiently flat.